A couple of weeks ago, someone asked in a post titled "timing vacuum/holley 600cfm" about where to connect a vacuum advance line.

The consensus was that it whould be connected to a manifold vacuum source, rather than a ported vacuum source. I have heard both ways, but manifold vacuum was the usual answer.

I was rereading the August 2000 Super Chevy and this was written on page 82:

"For good street driveability, your distributor's vacuum advance should be connected to "ported vacuum", which is drawn from a source in the carburetor above the throttle plates. This results in increasing vacuum as the throttle is applied. the function of vacuum advance is to provide timing advance in low-rpm acceleration conditions for smoothness and improved driveability. The alternative, manifold vacuum, is taken from a source below the throttle plates, resulting in decreasing vacuum as the throttle is applied. If your distributor is connected to manifold vacuum, full advance may be applied at idle. Then as throttle is applied, vacuum decreases and timing advance is decreased, causing poor engine performance and acceleration at low speeds."

Can either be used depending on application? Were the Team Camaro techies incorrect? Can Ignitionman save the day and give us the correct answer?

I am always surprized to hear people using manifold vacuum for advance. I was always taught that you want more advance when you hit the gas and that is when ported vacuum kicks in. Otherwise you would have too much advance at idle. No??? I just saw an article in one of the other magazines and it said the same thing that you quoted in Super Chevy.

problem with ported vacuum is slow to advance with a high liftcam little or no vacuum,vacuum at base plate increases with rpm. so i feel if you run a stocker, ported is fine,otherwise full .


just my thinking.

------------------
My68Camaro (http://hometown.aol.com/Dougs68Camaro/index.html)
Doug G.
68 Camaro
406 ci.

You people do really do believe what some fuzz-head prints in magazines over what the actual engine is asking for.

I'll hold out just a bit more, so all the hype and magazine article believers can post.

Nothing against anyone here, but this really needs to be set straight. Post away, please.

I will stick with manifold vacuum. If the answer is ported vacuum, why would they always tell you to disconnect the vacuum advance before setting the timing? A ported source generates no vacuum at idle.

Everyone, post some more so I-man will give us the real scoop.

[This message has been edited by gheatly (edited 08-08-2000).]

Even in the Edelbrock setup instructions it states what IgnitionMan has been saying all along. Ported for emission.......full for non-emission.

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68 RS, Ash Gold,pwr windows,Hounds tooth

Does it matter which manifold vacuum port? The one on the carburetor base plate or on the intake runner?

------------------
Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc

In my ZZ4 directions it says to use ported. GM may make cars that break all the time but I have to believe them when they tell me which vacuum port to use!! Take is easy on us Ignition man! : - ) By the way the full name for ported vacuum is "TIMED ported vacuum"....

Suldog - Wow!!You got directions with your ZZ4? When I bought my ZZ3 several years ago, all I got was a warranty sheet with a bunch of disclaimers.

CA420, I figured that's what the answer was going to be.

[This message has been edited by gheatly (edited 08-08-2000).]

Ignitionman,

Could you please give us your opinion on this? I learned the ported vacuum for vacuum advance method. I worked for a WD and the rep for Accel and Mallory both said ported vacuum; of course this was almost 20 years ago! A customer had a stumble off-idle which was cured by a switch from manifold vacuum to ported vacuum for his distributor. So in my younger years this was how it was.

Now it makes since to me that manifold vacuum may be correct in a lot of cases, especially a high-performance engine with a larger camshaft making less vacuum. What little advance you may lose under acceleration would be compensated by your centrifugal advance. It would help prevent pinging under light and full throttle. It also may allow a little better gas mileage at cruise conditions; not a major point with most high-performance guys, but a nice side benefit anyway.

Bottom line is that I see the reason for both but am not positive where the line is for using one method versus the other. Ignitionman, please let us know your theory on this subject. Am I way off base or what?

Jody

gheatly,

I actually did not recieve directions at first. I called the place where I bought it and they mailed me a COPY of the manual. What I don't understand is all the conflicting information in the instructions and what information I have found on the net. Instrutions say idle at 650. Everywhere else says 800. No way this motor will run at 650. I did expect to get piles of papers with the engine. Some ZZ4 decals would have been nice!!!

First i dont have a hi-perf engine and also non-emission. I have an original 1 owner car, nothing has been changed and my vacuum advance is connected to "ported vaccuum". This is how it left the factory.

Gheatly- I disagree with you on a "ported vacuum source" having no vacuum at idle. I just checked my vacuum advance, disconnected the line while idling and if i put my finger over the end of the hose i can feel the vacuum. I also verified this when i was setting my timing, set it with vacuum disconnected then reconnected vacuum and timing changed several degrees.

I-man we are patiently waiting for your words of wisdom.



------------------
68 Ash gold RS 327
factory AC,PS,PB
P-O-P

joe68rs,

The level of vacuum draw by a ported vacuum source and the RPM at which vacuum begins is supposedly engineered for a specific application. Therefore, different carbs and cars will have different ported vacuum flow characteristics.

My "How to Super Tune Holley Carburetors" book says that Holleys do not generate vacuum at the ported source at idle. I have never used ported vaccum and have always plugged the nipple in the metering block. My car is sitting in a paint booth, so I can't confirm this in person (maybe someone else can). I thought I read in a previous post that I-man had worked for Holley, so maybe he will clear this up.

I-man...Hello...Are you out there????

see a post in the troubleshooting section, dated7/11/00 "Rat not happy with vac advance"
TTFN

I think Ignition man is just waiting for the right moment and then he is going to rip us a new... well he is going to rip us apart! Ahh the suspence is killing me...

No ripping of anyone will take place, just the proper exlanation of the workings of the vacuum advance.

There are still more posters out there who haven't chimed in yet, get to it, please, so I can get to the answers.

I'd run whatever my engine responded to.
My next street engine, I'm going to try the manifold vaccum connection.

If I had a dieseling problem I might lean toward the ported vaccum hoping to keep the combustion chambers cooler. I'm not shure if it would make a difference.

I'd be interested in weather the engine would return to idle as quickly with the manifold vaccum. It seems like the ported vaccum might work a little better in that respect.

As Ignitionman prefers the direct vaccum, I'd use that as my starting point and only change if I had a problem.

David

------------------
Check my web page for First Gen Camaro suspension info:
David's Homepage (http://ourworld.compuserve.com/homepages/David_Pozzi/)
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer

[This message has been edited by davidpozzi (edited 08-09-2000).]

well with my 406, i had nothing but problems with advance (guess due to cam;low vacuum)went to full vacuum and BIG improvement,no more missing,stumble,or hesitation. totally different motor.<--- this was about 8-10 years ago.since then motor was gone through twice for new gaskets and clearence checks.

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My68Camaro (http://hometown.aol.com/Dougs68Camaro/index.html)
Doug G.
68 Camaro
406 ci.

OK, I read the L O N G post in the "Troubleshooting" section that Mat mentioned above. If I had seen it, I would not have made this post since, IMO, I-man did a good job of explaining the issue there. However, based on the number of responses to this post, a number of other people didn't catch the previous post either.

I-man, from a practical standpoint, I see your reason for using manifold vacuum and I agree with you. However, I am having a hard time understanding from a more theoretical standpoint.

It would seem that as RPM increases, an engine would need more and more advance because the amount of time for the combustion event to take place decreases. So I would think there would be a direct relationship between the amount of advance and engine RPM. Generally, this is the case, but only up to the point where mechanical advance is limited (2,500 RPM or so was mentioned in the previous thread).

Could it be that because high performance engines are more efficient (i.e., the intake charge is denser) at higher RPMs and since a more dense mixture burns faster the additional advance is not required?

Perhaps the quality of today's poo-poo, oxygenated, homogenized, vitamin zero pump gasoline has something to do with it.



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Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc

IgnitionMan - I have already benifited from manifold vacuum (383 ci w/auto trans, smoother idle, less pull against the converter etc) My problem, (not the secondaries from my other post) is I can't read my timing at idle... It bounces all over... I have set the total mechanical at 32degs and added 6deg with the adjustible vac canister. Idles between 8-900 in park and 6-700 in gear. What could be causing the timing to bounce at idle? I haven't found any vac leaks. It's a GM HEI that has been curved to be all in before 3000.... Converter stall is around 2100.

Maybe Al can add this to the "Tech Reference" when you post your "exlanation of the workings of the vacuum advance"

------------------
...Dennis
Topless'69 (http://home1.gte.net/ddunio/topless69)
Camaros Limited Nor-Cal (http://camaroslimited.com)
rsss.fun@gte.net

Hey,IgnitionMan, where's the answer. We're waiting.

I always wondered about those magazine guys I've seen them spread info at times that I thought was questionable.
I like to do what my engines like and so far my small blocks like manifold vacuum.
I let the mechanical curve do the advancing when I'm on it (low vacuum high engine load)I don't want the vacuum adv dialing in more timing when I load the engine as ported does cause it'll ping.
With high vacuum and low engine load (cruising) the engine likes the extra advance. They idle smoother and get better gas mileage, thats all I want vacuum advance for anyway.


[This message has been edited by Hp2x426 (edited 08-15-2000).]

[This message has been edited by Hp2x426 (edited 08-15-2000).]

Last post should read 40-45 degrees sorry, don't what happened to the edit.

Gee, 69er, I only have a business to keep going, and not all the time in the world to read magazines and articles by dyno kings, so if it comes to a little time in my response vs posting, I'll keep the time and the business going, thanks.

Now, for the rest of us: Vacuum advance.

Ported vacuum advance was, in fact, used by many makers of performance and stock engines in the past. Reason, the fuels were so good that the burn temperatures of the combustion chambers actually lowered as upper rpms were reached, and the added timing from ported was needed to heat the burn properly. In those days, the amount of degrees added in the cannister were fairly large, with some engines having 20+ degrees, but this number was usually not attained, as the total amount of vacuum added by the ported port was lower than that needed to get the vacuum advance to pull all the way in.

Today's pump vended fuels, and their additive packages, or lack thereof, have just the exact opposite of the earlier fuels, they always burn much too hot, even stuff like the Sunoco 280 have this problem. Add vacuum advance into the lace most succeptable to detonation, upper from mid to upper rpm ranges, and you can take detonation to the bank, you're gonna get it.

The only time today that ported timing is beneficial, to a point, is on Exhaust Gas Recirculation designed engines. These engines recirculate partially burned gasses from the exhaust system into the intake tract again to be re-burned and reduce emissions output, and the partially burned fuel needs the added combustion chamber heat to more completely burn the exhaust fuels recirculated.

In today's non-EGR performance engines, stock to performance, we do not require any upper rpm added timing for burn temperature control, in fact, it is detrimental to proper running.

So why use a vacuum cannister at all? Well, there is a simple silver lining to vacuum advance, engines as we are working with like timing at idle in the 18 to 24 degree range, but this isn't all that easy to do with some parts packages. How do we do this? Well, first, we need to think of the vacuum advance as not that, but an idle timing supplement to the initial timing.

As we now know, the idle timing can now be raised to a decent level at idle, to help with a stable idle, pull against a bigger than normal cam, pull against a converter and help cool the engine better from more complete idle fuel control. There is a catch, though, too many degrees of supplemental idle timing can be very counter-productive.

If we use the 24 degree figure, we can do things like set the initial timing at, say, 14 degrees, starts easily, then add the 10 extra with the cannister. OK, how do we get the 10 crankshaft degrees into the 20 degree cannister? We use either a Crane scroll plate to stop the advance pin that conects the diaphragm to the point/pickup plate to restrict plate movement down to the number of degrees we wish different from the static initial timing and the total idle timing.

OK, now that we have the number of degrees restricted, do we want to be able to change the vacuum level the canister comes in goes out with? Yes. We do this in two different ways, we look at the information of the stock cannister pull rate, and select the proper one for our vacuum levels, or we add the adjustable cannister from Crane, with the scroll plate that comes with it. Adjust away, you will find the correct amount of added timing for idle to make your engine run correctly.

Well, doesn't the idle added vacuum timing stay in the system when the gas pedal is hit? No, actually. It is immediately dropped as the off-idle vacuum goes away, and the centrifugal advance acts as usual . Only 180 on this is an engine that has no idle vacuum from a realy radical cam duration, and then, the vacuum actually rises after the engine is brought off idle. We just wouldn't use any kind of vacuum advance on an engine this radical anyway. Static idle timing would also be jumped up to 18 or so with the redical engine anyway, to try to stabilize the idle quality. Once again, adding initial timing helps the idle, just depends what you have available to help.

All HEI distributors manufactured for GM vehicles as production ignitions have way too much advance, mechanical degrees and vacuum degrees, and usually cause mucho problems in our performance and stock engines until they are set up correctly, and have full intake manifold vacuum sourced.

99 percent of all vacuum advances have way too many degrees of timing available and need to have the total number reduced to work with full manifold vacuum, as the manifold vacuum will pull all the degrees available into the cannister, not like the partial vacuum pull available with ported vacuum sources does at rpms.

Now, to dispell some myths about dyno tests and ported vacuum advance. Dynos always use more jetting to keep the chamber, engine and water temps cooled off, and ported timing just counteracts the rich mixtures of the rich jetting meeded for the dyno. In all the experiences I have had with crate and built performance engines, ported vacuum advance has been simply not productive, no matter what the engine did on the dyno.

If you are forced to use ported vacuum for your supplemental timing surce, then the curve and initial are not correct, and/or the total amount of vacuum supplemental timing is too large foruse as an idle supplemet from full intake manifold vacuum.

Now, 69, is this suficient, or do you need pictures and a coloring book?

Thanks Dave. Now, everyone print this out and save for future reference.

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Leo Paugh
MCC #017
www.clark.net/pub/chevelle/mcc.htm (http://www.clark.net/pub/chevelle/mcc.htm)
the bitterness of poor quality remains, long after the sweetness of the low price is forgotten

IgnitionMan,

That was a very well stated post. I do hope Al can find the time to add it to the "Tech Reference" area.

We all know you are very busy. Did 69er really need the tongue lashing though? No reply is needed, it's just food for thought!



------------------
...Dennis
Topless'69 (http://home1.gte.net/ddunio/topless69)
Camaros Limited Nor-Cal (http://camaroslimited.com)
rsss.fun@gte.net

Dennis, he fully started it, not me. I have not slept for over 70 hours,did a car show, then a swap meet, then came home and did a full fix on neighbor's Suburban BB so he coulkd go to work and make the money for his house payment and food for his kids, and then I get the "When", with the attitude here, I don't need that. I still have at least 20 hours more today to go. 11 more distrbutor conversions today, take my doesn't drive sister around tomorrow, no sleep, didn't need the flip remark.

One worst thing about the net, we never see each other, we never hear any inflection in the voice to determine what is joke or serious words, must go by exactly the way it is written, read it as a "I don't really think he will do it, he must be bull****ting:. Ain't so.

Don't really care how he feels, read like a challenge, that's all. I gave it right back to him, that's life.

(hahahaha) leaves no doubt in anyone's mind about intention of post, though. I ain't (hahaha)-ing here.

Sorry, Dennis.

Ignitionman, I would just like to say thanks for sharing the knowledge that you have. It's guys like you that help alot of us figure out what we are doing or needing to do. Its nice to have people around who have the answers. Thanks agian.
Mike McK
67 SS/RS 396
macs67ssrs@hotmail.com

You know you didn't have to be smart about it Ignition man. I was not trying to be funny, I actually wanted to know so I could have a better understanding of how it works. I WON'T bother you again. Bob

You know you didn't have to be smart about it Ignition man. I was not trying to be funny, I actually wanted to know so I could have a better understanding of how it works. I WON'T bother you again. Bob

Two times, even. I got the picture that you weren't making a joke of it. I really did.

Great googly moogly.......I'm going to let my 8 year old son start posting on this board, it might raise the level of maturity a few notches.......good grief. KZ

My my my, it just never stops does it?

What total bs...this place is polluted now, too.

IgnitionMan,
An impressive post, I'm saving it.
Don't want to seem ungrateful by not posting a thanks, so here it is.

But, could you..... Never mind.
David

Look, everybody, we are all saved, now, Larry's here.

I'm with David Pozzi on this. Can we close this post before it gets out of hand?

------------------
67 Camaro LS6 454/TH400/12bolt 3.73
1989 TransAm 5.7L WS6 W/all the options

I'd like to see it closed as well, just leave it for the information only, and not for the flames I get every time I tell it like it really is. As far as I am concerned, it's closed.

Ig-man thanks tons for the acknowledgement!
But I'm not here to save anyone. See!! There's something you don't know.

it depend on the appliciton,steet ported strip none

Looks like at least one poster didn't read any of this topic except the initial question, and doesn't realize vacuum advance tech has changed from 10 years ago.

It really is time to close it. I mean boys will be boys, but enough is enough.

Ignition Man...admittedly you have given a very thorough explanation of ported vacuum, and yes you have a wealth of information to share. I can give credit where its due.

However, my advice is this, if you want the respect of others, don't cram it down their throat, and lose the insults....nobody likes it....especially not yourself.

If you feel that I am incorrect and would like to prove me wrong, I have a recommendation for you....change your user name and see what happens.

If you change your tone of writing, and use a different name, maybe some of this will stop, and give you a chance for a fresh start.

And please, for the love of God, do not read this as an offensive thread, because I am certainly not willing to entertain a shouting match. Just take it as friendly advice.

BBD

[This message has been edited by Big Block Dave (edited 08-23-2000).]

Hey, BBD, not a chance of changing my username, and I was right on target about the poster, he didn't even read the rest of the posts, just posted.

Who cares about respect and being one of the boys here, I really don't care about that, BUT I tell it like it is, take it or leave it. You won't EVER get incorrect info in my posts, and I won't sugarcoat it either, life's just too short to do that kind of thing. You will only get good and true info from me, take it or leave it, attitude included.

No-body has to respect me for a proper, correct answer to benefit from that correct answer's solution to a problem if it fixes that problem right.

Sorry you feel different about your posts than everybody else's, you have made some stinkers as well, most of your humor-isn't, a few of yours are very beneficial, though. NOTHING PERSONAL TWORDS YOU.

I've been fixing other people's screw-ups for all my life, without credit, almost all the time without pay and after some "noted expert" has made a total mess out of it, so what, that's life. I just want it done right, that's what I post, and don't really expect even a thank you for it. As long as it gets fixed right is thanks enough, even silent thanks.

If you decide to advise doing it wrong, I'm gonna call you on it, for sure. Don't like that, your problem. Don't like the attitude, still your problem.

[This message has been edited by IgnitionMan (edited 08-23-2000).]

<BLOCKQUOTE>quote:</font><HR>Originally posted by IgnitionMan:

Sorry you feel different about your posts than everybody else's, you have made some stinkers as well, most of your humor-isn't, a few of yours are very beneficial, though. NOTHING PERSONAL TWORDS YOU.
[This message has been edited by IgnitionMan (edited 08-23-2000).]<HR></BLOCKQUOTE>

Not taken personally at all, but I tell you what Ignition Man, you come to NY and meet me in person and I'll make you laugh so hard you'll sh!t your pants o.k? Try not to take everything so seriously and have a little fun from time to time allright? And bring a change of shorts.


[This message has been edited by Big Block Dave (edited 08-23-2000).]

This thread ceased to include any meaningful input about ported vs. manifold vacuum about 20 posts ago. Give it a rest guys and move on.

------------------
Shawn Peterson
1969 Z28, 406SB
http://www.geocities.com/corsican69

Why does GM tell me to use Ported Vacuum on a brand new ZZ4? Are they wrong? Although I'm losing lots of respect for GM, I have to think that they are giving me the correct instructions.

Re-read the informaiton I posted further up, the answer is there.

I read the post of the I-man. And I've got to say I didn't understand a word of it. I guess I'll just run a vac. hose to my mouth and suck real hard when I take off

<BLOCKQUOTE>quote:</font><HR>Originally posted by zgator:
I guess I'll just run a vac. hose to my mouth and suck real hard when I take off<HR></BLOCKQUOTE>

....No comment http://www.camaros.net/forum/rolleyes.gif

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67 Camaro LS6 454/TH400/12bolt 3.73
1989 TransAm 5.7L WS6 W/all the options

Hey Zgator, what's there not to understand? Aren't you fluent in Greek? I'm not going to comment on that other remark, I've got about 10 good lines, and can't decide which one is best........ http://www.camaros.net/forum/smile.gif KZ

The hate is swelling within you. Take your lightsaber and strike me down and your journey towards the darkside will be complete.....Oh sorry, all this baiting and taunting reminded me of a movie I watched earlier tonight with my 8 year old.

Smile dammit!

hey igniton man let me know when you put a car in 7 sec range. then tell me what kind of vacume you run on it.

Actually, I do work for a bunch of 4 second nitro people, and I run a low 6, very high 5 second car just for fun, so I've already gone past your 7 second requirement. And, yes, all those particular vehicles are radical enough to not use vacuum advance.

Now, what did you have in mind to start and cause trouble on this topic here? Whatever it is, nobody here needs it, including me.

Enough already.......Someone close this thread before I blow chunks........KZ

It's prety obvious IMAN NOWS a little on the subject. But my god. Moderator please keep the IMANS INFO, and delete the rest.

30+ year old car, 20 year old motor....it worked then, and works now. nuff said.

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My68Camaro (http://hometown.aol.com/Dougs68Camaro/index.html)
Doug G.
68 Camaro
406 ci.

Doug, timjes and engine technology change, and so has the useage of vacuum advance added timing. Respectfully to you, ported vacuumm is completely detrimental for today's useages, either stock and/or moderately modified engines, unless the engine has EGR.

Of course, very serious hot rod engines don't make enough vacuum either ported or full, to run a vacuum advance anyway, so on those engines only, a full mechanical only curve is workable, but only on those radical engines.
Dyno testing is mis-leading, requires things in timing and carburetion that simply don't work in the real world driving, engine in the chassis situations.

Even new GM crate engines, ones that are dyno developed with ported vacuum advance, will benefit from altering the total vacuum advance downwards, once off the dyno and used in real works driver situations, with the cannister then connected to a full manifold vacuum source to aid in idle and low speed driveability, mileage and added power, instead of losing power, driveability, mileage and potential over-heating from over-advancing of the timing at higher rpm levels from ported sources. Proven many times over, it is fact.

Sorry, it just doesn't work like it used to, there's different rules now.

DEVIL'S LAKE, nothing up anywhere. When you make a statement as to how fast you are, like you did, "when you get to the 7's", it might be beneficial to you to investigate just how much faster (or slower, and that ain't me) the person you are picking a beef with really is than you. I ran in the 7'teens to 0's in 1975, have been faster than that for over two decades. Then, you can make a serious, concise, informed post to start your trouble. Thanks.



[This message has been edited by IgnitionMan (edited 08-27-2000).]

Well... given all of the coverage this post has received, I guess I should admit that I run a mechanical advance distributor. So, none of the great info I-man provided applies to my current engine combination.

BTW, I was playing around with the father-in-laws original Power-Pack 283 in his '57 Chevy and the vacuum canister was connected to MANIFOLD vacuum.

What other seemingly innocent, but controversial topic can I post next???

------------------
Hugger Orange & white 69 Camaro with supercharged 350, Tremec TKO, and 3.73 12-bolt

See my website updated 8/13/00 at:

www.geocities.com/gheatly (http://www.geocities.com/gheatly)

[This message has been edited by gheatly (edited 08-28-2000).]

I think you did great on this one.

Most entertaining indeed.

Hey I-man,

How come you stopped posting over at nastyz28.com? Too many people over there show you up?

Act your age and stop taking everything so serious.

Don't worry about replying, as I'm blessing you and this site with my absence... at least for a while. It's a shame that a few individuals, however knowledgeable they are feel they are GOD and that their fecal matter doesn't stink!

Later All...


------------------
Robert Yates for President!!

[This message has been edited by YatesFan (edited 08-29-2000).]

I am working off my laptop unitl my bigger computer is finished, server install so I can work my own web business. So, my bookmarks are not same, and I don't get around as much.

And, there, FORD FAN, I never got "shown up" as you say, there. I spend my time on the best of the sites, not the ones with people who just want to cause trouble. I never saw any constructive posts you made that helped anybody there.

Looked like a big high school clique there.
Leave Robert Yates in NASCRAP, he belongs there, not president.

[This message has been edited by IgnitionMan (edited 08-29-2000).]

We prefer not to close postings, but as mentioned earlier, this one has ran out.

I just dealt with this situation.

69 350 mild cam upgrade everything else base. manifold from a 74 carb from 75 350.
My problem was off idle stumble with the quadrajet. Got a rebuilt "carter" quad to see if that would help.
carter quadrajet looks almost exact but had a little vacuum at idle on the distributor advance port. The old rochester didnt have vacuum at idle but did as soon as you gave it gas.

The carter quadrajet which was supposed to be exactly the same carb numbers wise was totally different as far as the vacuum from that port at idle. This totally took me by surprise.
with the non ported like vacuum I had a super bog and the secondaries died relative to the way it ran with the rochester.
So, I put back the rochester cleaned it up and put in new plugs and its almost back to normal and makes the quadrajet roar and stumble is almost gone.I may advance the timing a hair yet and tighten up the air valve plates to get rid of it.
The point of all this is that I put it on direct manifold vac with the rochester to see what would happen ( different port) and it stumbled bad mid acceleration and had no killer roar when you nail it.

So, my conclusion is each carb ,even ones that are supposed to be the same may be different and each setup either likes ported or direct better, you just have to see which one and its very easy.

just my two cents on this topic!